1. Field of the Invention
This invention pertains to a data link terminator for a plurality of time-divisionally multiplexing digital lines.
2. Description of the Related Art
With recent advances in ISDN (integrated services digital network) technology, time-divisionally multiplexing digital lines are becoming increasingly important.
With a time-divisionally multiplexing digital line, a technique is required for efficiently transmitting control data such as interoffice maintenance information, as well as communication information and call control information. Such an interoffice digital path is generally called an interoffice data link or simply a data link.
A representative data link configuration uses a frame synchronization bit (or simply F bit).
For instance, if ISDN primary rate digital interface has a transmission rate of one point five four four megabits per second (1.544 Mbits/s), one (1) frame comprises a total of one hundred ninety-three (193) bits. More specifically, a frame has twenty-four (24) time-divisionally multiplexed channels each having eight (8) bits and an F bit at the head end. A plurality of frames, each frame discerned by its F bit, are transmitted in a string. A string of frames is called a multiframe. A multiframe comprises twenty-four (24) frames on a time-divisionally multiplexing digital line or sixteen (16) frames on an intraoffice link, for example.
This transmission system allows bit data for a data link to be transmitted, as well as a pattern sequence for frame synchronization and a bit sequence for error correction. Because only the F bits (frame synchronization bits) of a half of all the frames, e.g. odd numbered frames, are extracted in demultiplexing frames as data links, a data link has a transmission rate of four kilobits per second (4 Kbits/s).
Conventionally, a switching office has its terminators, each equipped with an appropriate microprocessor, because respective digital lines extract and process control data transmitted over the respective digital lines using the above data links.
However, this technique has the problem that many terminators are required for processing data links, commensurately with the number of digital lines accommodated by a switching office, thus making the switcher expensive.
Since such a data link transmits control data according to a predetermined procedure based on a protocol such as the LAPD (Link Access Procedure by D channel), the microprocessor of a terminator needs to execute a program enabling data to be exchanged according to this procedure. However, this technique requires the microprocessor processing procedure to be changed at all digital line terminators when the protocol changes at some future point, making the technique quite cumbersome.
As described above, the prior art has the problem that the same processes must be duplicated for respective lines even though the same processing protocol is used for control data on the data links of respective digital lines.